Star-branched polymers are of substantial commercial interest due to their markedly different dilute solution behavior compared to linear, random-coil counterparts. In addition, they have important commercial applications, most notably as viscosifiers in lubricating oils, due to their superior resistance to shear-induced thermal degradation. With the exception of classical condensation polymerization systems and closely related ring-opening polymerizations, star-branched polymers have been produced by three different methods using, in most cases, living anionic polymerization. The most popular method, termed the "linking" method, involves the initiation of monomers such as styrene, butadiene, or isoprene with an organolithium initiator to produce living polymeric monoanions that are subsequently reacted, in exact stoichiometric proportions or in slight excess, with a multifunctional linking agent to form star-branched polymers. The advantage of this method is that the number of arms per molecule within a given sample is invariant and precisely controlled by the functionality of the linking agent.
Star-branched polymers have also been synthesized by sequential addition of a di- or polyfunctional vinyl compound such as divinylbenzene to living, monofunctional polymeric anions. This second general method which may be described as an "arm-first, core-last" method, is particularly suited to the preparation of star-branched polymers with many arms. The first step of the synthesis involves the generation of living polystyryllithium. A small amount of difunctional vinyl compound such as divinylbenzene is charged to the reaction, and this initially forms a short block segment of divinylbenzene with pendent vinyl groups. The last step of the mechanism involves inter- and intramolecular reactions (microgel formation) of these vinyl groups with living anions to give a star-shaped polymer in which the arms are radially attached to a microgel core.
The third general method for producing star-branched polymers, termed the "core-first, arm-last" method, involves the use of a multifunctional initiator that is either prepared externally to the polymerization reaction or in-situ, just prior to polymerization of the arm-forming monomer. Externally prepared multifunctional anionic initiators have been generally less successful due to poor solubility of low-molecular weight polyanions, and the few reported examples require polar-solvent conditions.
A successful variation of this method, which is also closely related in principle to the arm-first, core-last method, involves the in-situ formation of a plurifunctional anionic initiator by the reaction of an excess of difunctional vinyl compound such as divinylbenzene with butyllithium in dilute solution. This reaction produces a population of soluble gel particles, each of which is coated with a number of carbanions which are capable of initiating a subsequent charge of monomer to produce a star-branched polymer molecule.
Recent developments in the field of living carbocationic polymerization have enabled the synthesis of various vinyl ether star-branched polymers by the arm-first, core-last method. An HI/ZnI.sub.2 initiating system has been used at -40.degree. C. in toluene to produce living poly(isobutyl vinyl ether) cations to which a small amount of divinyl ether such as that shown below, ##STR1## was added to produce star-branched polymers. Polymers ranging in arm number from 3 to 60 were produced by this method. The Hi/ZnI.sub.2 initiating system provides living character to the isobutyl vinyl polymerization through the intimate interaction between growing chain ends and their binary I.sup.-. . . I.sub.2 counterionic moieties which render the chain ends reactive toward propagation but unreactive toward higher energy chain transfer and termination reactions. This method of living carbocationic polymerization has been termed "stabilization by a suitably nucleophilic counterion," and has been successfully used to produce living carbocationic polymerizations of monomers such as vinyl ethers, p-methoxystyrene, and N-vinyl carbozole, which form highly stabilized carbocations.
A more recent development, involving the addition of Lewis bases such as ethyl acetate, dimethyl sulfoxide, and dimethyl acetamide to the reaction medium of a carbocationic polymerization, has been found to provide living characteristics to the cationic polymerization of monomers which form relatively unstable carbocations, such as isobutylene (IB) and styrene, in addition to those monomers which form relatively stable carbocations. This method of living carbocationic polymerization has been termed "electron donor mediated cationic polymerization," since the Lewis base or a Lewis base:Lewis acid (coinitiator) complex is believed to provide stabilization to growing polymer chains. The most obvious manifestation of this stabilization is a dramatic decrease in the overall rate of polymerization, which is most often attributed to the formation of an equilibrium between a large number of dormant (reversibly terminated) chain ends and a small number of active chains.
An improved Lewis base mediated polymerization system for the living carbocationic polymerization of isobutylene (IB) consists of a di- or trifunctional cumyl chloride-type initiator in conjunction with TiCl.sub.4 as coinitiator and pyridine as an externally added Lewis base (electron donor) in a 60/40 hexane/methyl chloride solvent system at -80.degree. C. The improved system has been utilized to produce telechelic ionomers possessing a narrow MWD between ionic groups, poly(styrene-b-isobutylene-b-styrene) (S-IB-S) block copolymers, and S-IB-S block copolymer ionomers.